The present invention belongs to the field of organic electronics and magnetoelectronics and relates to a method and a device for injecting and transferring spin-polarized charge carriers, particularly in organic semiconductors.
Control of the spin polarization (SP) of charge carriers injected into metallic, semiconducting or insulating materials allows to introduce and control a new degree of freedom in different electronic devices. A new type of electronics, known as magnetoelectronics, has been developed on this basis and has a decisive role in the technology of computers, multimedia and communications.
Many types of processors and memory elements for computers are in fact based on the property of the devices to change their electric resistance according to the orientation of the spins of the charge carriers, thus allowing to read the magnetic information by means of a simple measurement of resistance [1].
The discoveries of materials which exhibit giant magnetoresistance (magnetic metallic multilayers, GMR) [2] and colossal magnetoresistance (perovskitic manganites, CMR) [3] lead to an essential improvement in this field due to the strong electrical signal that can be obtained in these materials even in low magnetic fields.
GMR materials, discovered in 1988, were used in the computer industry already in 1994.
Several electronic devices based on CMR materials have been proposed during the 1990s. These devices comprise both memory elements and various types of active elements, such as magnetic processors, magnetic transistors and hybrid transistors with inserted superconducting materials.
On the one hand, organic electronics has undergone a great development starting from the 1990s. Essentially, this relates to light-emitting diodes (LED) and FET transistors. At present, there are no devices in which the organic material would be used with an active element in magnetoelectronics. The use of organic materials in electronics has advantages, the most important being:
1. The possibility to deposit organic films with low-cost methods even on a very large deposition area. Differently from semiconducting films, which require ultrahigh-vacuum techniques, deposition of organic materials can occur even in ambient atmosphere.
2. The low cost of said organic materials.
3. The possibility to create flexible devices, for example various color displays.
4. The possibility to vary the colors in optical devices, so as to cover the entire visible spectrum.
5. The possibility to reversibly deposit organic films, which can be subsequently removed and replaced with other materials.
Accordingly, the development and efficiency improvement of organic electronic devices are of primary importance.
In order to produce magnetoelectronic devices it is necessary to have the possibility to create inside the active element of the device spin-polarized charge carriers. There exist various ways for spin-polarized charge carriers creation. For example, by illuminating the properly doped GaAs or a similar semiconductor with circularly-polarized light one excites triplet excitons with their spins aligned along the external magnetic field [4]. It is also known the use spin filters, i.e., devices across which can pass only charge carriers having a certain spin polarization [5]. However, such methods are complicated, expensive, and require the presence of magnetic field.
A third known method is the use of a magnetic material with intrinsic spin polarization. A large fraction of magnetic materials has a larger number of electrons whose spins are orientated parallel to the magnetic axis (N.uparw.). Usually, the difference between the larger number of electrons whose spin orientation is parallel to the magnetic axis and the smaller number of electrons whose spin is orientated otherwise (N.dwnarw.) is rather low. For example, in Ni this difference is 15-20%. However, there are some materials, termed half-metallic ferromagnets, whose electron spins are 100% polarized. These materials include chromium oxide, iron oxide and manganites of the type A.sub.1-x B.sub.x MnO.sub.3 (where A is a rare earth, i.e., La, Nd, et cetera, and B is a divalent metal, i.e., Ca, Sr, Pb, et cetera).
The magnetic order of manganites can be described as follows: the manganese atoms, which can have both 3+ and 4+ valence, have spins S=2 and S=3/2, respectively. Below the Curie temperature (which varies between approximately 300 and 400 K in these materials), these magnetic moments arrange themselves in a parallel configuration due to the electron exchanges between Mn(3+) and Mn(4+). Due to a strong Hund energy, the manganese sites accept only electrons whose spins are orientated like their own magnetic moment. Accordingly, delocalization affects only one half of all the electrons, i.e., those having spin parallel to the magnetization axis of the Mn ions; those with an antiparallel spin remain localized. As a result, 100% polarization of the spins of charge carriers (the delocalized electrons) is produced.
When two ferromagnetic electrodes with polarized spins come into direct or tunnelling electric contact, the total electric resistance depends on the angle of misorientation of their spins: for parallel spins, resistance is lowest; for antiparallel spins, resistance is highest. This is described in qualitative terms by the deGennes formula for the electron transfer probability T.sub.12 [6]: EQU T.sub.12 =b.sub.12 cos(.THETA..sub.12 /2)+const, (1)
where 1 and 2 correspond to the two ferromagnetic electrodes, b.sub.12 is a tunnelling constant, and .THETA..sub.12 is the angle between the magnetic axes of 1 and 2. If the external magnetic field is zero, the angle .THETA..sub.12 can have any value between 0 and 180.degree.. By introducing the magnetic field, both ferromagnetic electrodes assume the same oriention, the angle .THETA..sub.12 =0, and the value of T.sub.12 reaches its maximum (lowest resistance). This is the cause of negative magnetoresistance. Owing to very high negative magnetoresistance values (up to six orders of magnitude), manganites are known as Colossal Magnetoresistance (CMR) materials.
In recent times, the fact has been clarified that the value of magnetoresistance in CMRs is truly high (up to 6 orders of magnitude) only for electric devices of the tunnelling or point-contact type. Individual crystals instead exhibit a magnetoresistance of 2-3% [7]. Accordingly, the only magnetoresistance devices that have an application value are tunnelling or point-contact ones.